Control of Intermediates and Products by Combining Droplet Reactions and Ion Soft-Landing.

Despite being recognized primarily as an analytical technique, mass spectrometry also has a large potential as a synthetic tool, enabling access to advanced synthetic routes by reactions in charged microdroplets or ionic thin layers. Such reactions are special and proceed primarily at surfaces of droplets and thin layers. Partial solvation of the reactants is usually considered to play an important role for reducing the activation barrier, but many mechanistic details still need to be clarified. In our study, we showcase the synergy between two sequentially applied "preparative mass spectrometry" methods: initiating accelerated reactions within microdroplets during electrospray ionization to generate gaseous ionic intermediates in high abundance, which are subsequently mass-selected and soft-landed to react with a provided reagent on a substrate. This allows the generation of products at a nanomolar scale, amenable to further characterization. In this proof-of-concept study, the contrasting reaction pathways between intrinsically neutral and pre-charged reagents, respectively, both in microdroplets and in layers generated by ion soft-landing are investigated. This provides new insights into the role of partially solvated reagents at microdroplet surfaces for increased reaction rates. Additionally, further insights into reactions of ions of the same polarity under various conditions is obtained.

LASER IONIZATION MASS SPECTROMETRY AT 55: QUO VADIS?

Laser ionization mass spectrometry (LIMS) was one of the first practical methods developed for in situ analysis of the surfaces of solid samples. This review will encompass several aspects related to this analytical method. First, we will discuss the process of laser ionization, the influence of the laser type on its performance, and imaging capabilities of this method. In the second chapter, we will follow the historic development of LIMS instrumentation. After a brief overview of the first-generation instruments developed in 1960-1990 years, we will discuss in detail more recent designs, which appeared during the last 2-3 decades. In the last part of our review, we will cover the recent applications of LIMS for surface analysis. These applications include various types of analyses of solid inorganic, organic, and heterogeneous samples, often in combination with depth profiling and imaging capability.

Direct functionalization of C-H bonds by electrophilic anions.

Alkanes and [B12X12]2- (X = Cl, Br) are both stable compounds which are difficult to functionalize. Here we demonstrate the formation of a boron-carbon bond between these substances in a two-step process. Fragmentation of [B12X12]2- in the gas phase generates highly reactive [B12X11]- ions which spontaneously react with alkanes. The reaction mechanism was investigated using tandem mass spectrometry and gas-phase vibrational spectroscopy combined with electronic structure calculations. [B12X11]- reacts by an electrophilic substitution of a proton in an alkane resulting in a B-C bond formation. The product is a dianionic [B12X11CnH2n+1]2- species, to which H+ is electrostatically bound. High-flux ion soft landing was performed to codeposit [B12X11]- and complex organic molecules (phthalates) in thin layers on surfaces. Molecular structure analysis of the product films revealed that C-H functionalization by [B12X11]- occurred in the presence of other more reactive functional groups. This observation demonstrates the utility of highly reactive fragment ions for selective bond formation processes and may pave the way for the use of gas-phase ion chemistry for the generation of complex molecular structures in the condensed phase.

Binding of saturated and unsaturated C6-hydrocarbons to the electrophilic anion [B12Br11]-: a systematic mechanistic study.

The highly reactive gaseous ion [B12Br11]- is a metal-free closed-shell anion which spontaneously forms covalent bonds with hydrocarbon molecules, including alkanes. Herein, we systematically investigate the reaction mechanism for binding of [B12Br11]- to the five hexane isomers yielding [B12Br11(C6H14)]-, as well as to cyclohexane and several hexene isomers (yielding [B12Br11(C6H12)]-) using collision-induced dissociation (CID), infrared photodissociation spectroscopy (IRPD) and computational methods. CID of the different [B12Br11(C6H14)]- ions results in distinct fragmentation patterns dependent on the structure of the hexane isomer. The observed fragmentation reactions provide insights into the addition mechanism of [B12Br11]- to hexane. Based on the observed CID patterns, we identified that either B-C bond formation through heterolytic C-C or C-H bond cleavages or B-H bond formation through heterolytic C-H cleavage occur dependent on the structure of the hexane isomer. Meanwhile, we observe identical CID spectra of adducts originating from isomers of C6H12. Spectroscopic investigations of adducts of 1-hexene and cyclohexane indicate the same product structure with an open C6 chain. Computational investigations evidenced that low lying transition states are present, which enable a ring opening reaction of cyclohexane when binding to [B12Br11]-.

Rational design of an argon-binding superelectrophilic anion.

Chemically binding to argon (Ar) at room temperature has remained the privilege of the most reactive electrophiles, all of which are cationic (or even dicationic) in nature. Herein, we report a concept for the rational design of anionic superelectrophiles that are composed of a strong electrophilic center firmly embedded in a negatively charged framework of exceptional stability. To validate our concept, we synthesized the percyano-dodecoborate [B12(CN)12]2-, the electronically most stable dianion ever investigated experimentally. It serves as a precursor for the generation of the monoanion [B12(CN)11]-, which indeed spontaneously binds Ar at 298 K. Our mass spectrometric and spectroscopic studies are accompanied by high-level computational investigations including a bonding analysis of the exceptional B-Ar bond. The detection and characterization of this highly reactive, structurally stable anionic superelectrophile starts another chapter in the metal-free activation of particularly inert compounds and elements.

Photoelectron spectroscopy of [Mo6X14]2- dianions (X = Cl-I).

Photoelectron spectroscopy and theoretical investigations have been performed to systematically probe the intrinsic electronic properties of [Mo6X14]2- (X = halogen). All three PE spectra of gaseous [Mo6X14]2- (X = Cl, Br, I) dianions, which were generated by electrospray ionization, exhibit multiple resolved peaks in the recorded binding energy range. Theoretical investigations on the orbital structure and charge distribution were performed to support interpretation of the observed spectra and were further extended onto [Mo6F14]2-, a dianion that was not available for the experimental study. The measured adiabatic (ADE) and vertical detachment energies (VDE) for X = Cl-I were well reproduced by density functional theory calculations (accuracy ∼0.1 eV). Corresponding ADE/VDE values for the dianions were found to be 1.48/2.13 (calc.) and 2.30/2.65, 2.30/2.62, and 2.20/2.42 eV (all expt.) for X = F, Cl, Br, and I, respectively, showing an interesting buckled trend of electron binding energy (EBE) along the halogen series, i.e., EBE (F) ≪ EBE (Cl) ∼ EBE (Br) > EBE (I). Molecular orbital analyses indicate different mixing of metal and halogen atomic orbitals, which is strongly dependent on the nature of X, and suggest that the most loosely bound electrons are detached mainly from the metal core for X = F and Cl, but from halide ligands for X = Br and I. The repulsive Coulomb barrier (RCB), estimated from the photon energy dependent spectra, decreases with increasing halogen size, from 1.8 eV for X = Cl to 1.6 eV for X = I. Electrostatic potential modeling confirms the experimental RCB values and predicts that the most favorable electron detaching pathway should lie via the face-bridging halide ligands.

"Solvent-in-salt" systems for design of new materials in chemistry, biology and energy research.

Inorganic and organic "solvent-in-salt" (SIS) systems have been known for decades but have attracted significant attention only recently. Molten salt hydrates/solvates have been successfully employed as non-flammable, benign electrolytes in rechargeable lithium-ion batteries leading to a revolution in battery development and design. SIS with organic components (for example, ionic liquids containing small amounts of water) demonstrate remarkable thermal stability and tunability, and present a class of admittedly safer electrolytes, in comparison with traditional organic solvents. Water molecules tend to form nano- and microstructures (droplets and channel networks) in ionic media impacting their heterogeneity. Such microscale domains can be employed as microreactors for chemical and enzymatic synthesis. In this review, we address known SIS systems and discuss their composition, structure, properties and dynamics. Special attention is paid to the current and potential applications of inorganic and organic SIS systems in energy research, chemistry and biochemistry. A separate section of this review is dedicated to experimental methods of SIS investigation, which is crucial for the development of this field.

Superelectrophilic Behavior of an Anion Demonstrated by the Spontaneous Binding of Noble Gases to [B12 Cl11 ].

It is common and chemically intuitive to assign cations electrophilic and anions nucleophilic reactivity, respectively. Herein, we demonstrate a striking violation of this concept: The anion [B12 Cl11 ]- spontaneously binds to the noble gases (Ngs) xenon and krypton at room temperature in a reaction that is typical of "superelectrophilic" dications. [B12 Cl11 Ng]- adducts, with Ng binding energies of 80 to 100 kJ mol-1 , contain B-Ng bonds with a substantial degree of covalent interaction. The electrophilic nature of the [B12 Cl11 ]- anion is confirmed spectroscopically by the observation of a blue shift of the CO stretching mode in the IR spectrum of [B12 Cl11 CO]- and theoretically by investigation of its electronic structure. The orientation of the electric field at the reactive site of [B12 Cl11 ]- results in an energy barrier for the approach of polar molecules and facilitates the formation of Ng adducts that are not detected with reactive cations such as [C6 H5 ]+ . This introduces the new chemical concept of "dipole-discriminating electrophilic anions."

Tuning of tetrathiafulvalene properties: versatile synthesis of N-arylated monopyrrolotetrathiafulvalenes via Ullmann-type coupling reactions.

An Ullmann-type coupling reaction was employed for the preparation of several N-arylated monopyrrolotetrathiafulvalenes with variable substitution patterns. Spectroscopic and electrochemical properties of the coupling products strongly depend on the electronic nature of the aromatic substituents due to their direct conjugation with the tetrathiafulvalene chromophore. The crystal packing of the arylated monopyrrolotetrathiafulvalenes is primarily defined by networks of C-H···X weak hydrogen bonds and short S···S contacts involving the tetrathiafulvalene moieties.

Light-controlled macrocyclization of tetrathiafulvalene with azobenzene: designing an optoelectronic molecular switch.

Macrocyclization between tetrathiafulvalene (TTF) dithiolates and bis-bromomethylazobenzenes/bis-bromomethylstilbenes is investigated under high dilution conditions. We show that macrocycles of different size can be formed depending on whether the (Z)- or (E)-isomers of azobenzene (AB) or stilbene are used. This represents the first example of a light-controllable cyclization reaction. The oxidation potential of the small, structurally rigid TTF-AB macrocycle is found to depend on the conformation of the AB moiety, opening the way for the modulation of redox properties by an optical stimulus. DFT calculations show that the out-of-plane distortion of the TTF moiety in this macrocycle is responsible for the variation of its oxidation potential upon photoisomerization of the neighboring AB bridge.

Rationally designed calix[4]arene-pyrrolotetrathiafulvalene receptors for electron-deficient neutral guests.

Four upper rim bis-monopyrrolotetrathiafulvalene-calix[4]arene conjugates 2a,b and 3a,b have been efficiently synthesized using a modular construction approach. The new compounds feature a molecular tweezer architecture with a quasi-parallel arrangement of redox-active tetrathiafulvalene (TTF) arms, which serve as the guest binding centers. Complexation studies using UV/vis binding titrations revealed a high affinity of the calixarene-TTF receptors for planar electron-deficient guests, leading to formation of deeply colored charge-transfer complexes in solution. The binding efficiency of the receptors depends on the flexibility of the calixarene scaffolds and the electronic nature of the TTF arms: the highest binding efficiency is shown by receptor 2b, featuring a highly preorganized molecular structure and an electron-rich TTF moiety.

2-(1,3-Di-thiol-2-yl-idene)-1,3-di-thiole-4-carbaldehyde.

The structure of the title compound, C7H4OS4, at 100 K has ortho-rhom-bic symmetry. In the crystal, tetra-thia-fulvalene mol-ecules form π-stacks along the a axis, with a stacking distance of 3.4736 (6) Å. Along the b axis, parallel stacks are inter-connected with each other through a network of weak C-H⋯O hydrogen bonds and short S⋯S contacts [3.4813 (7) Å]. Additional short S⋯S contacts [3.4980 (9) Å] join parallel stacks along the c axis.

5,11,17,23-Tetra-kis(chloro-meth-yl)-25,26,27,28-tetra-propoxycalix[4]arene.

The title calix[4]arene, C(44)H(52)Cl(4)O(4), displays the 1,3-alternate conformation with crystallographically imposed twofold symmetry. Four phenolic rings of the calixarene backbone are tilted into the calix cavity, making dihedral angles of 77.42 (2) and 77.71 (2)° with the plane of the four bridging methyl-ene C atoms. Pairs of opposite aromatic rings make dihedral angles of 25.16 (3) and 24.58 (4)° with each other. In the crystal, the calixarene mol-ecules pack with the formation of infinite columns along the b axis. The crystal packing shows a network of C-H⋯Cl contacts, which can be considered as non-classical hydrogen bonds.

2,3,6,7-Tetra-kis(bromo-meth-yl)naphthalene.

The title compound, C(14)H(12)Br(4), crystallizes with imposed inversion symmetry. In the crystal, the mol-ecules pack in layers parallel to (10). The layers involve two Br⋯Br and one H⋯Br contact. Between the layers, one contact each of types Br⋯Br, H⋯Br and Br⋯π is observed.

New Perspectives in the Noble Gas Chemistry Opened by Electrophilic Anions.

Binding of noble gases (NGs) is commonly considered to be the realm of highly reactive electophiles with cationic or at least non-charged character. Herein, we summarize our latest results evidencing that the incorporation of a strongly electrophilic site within a rigid cage-like anionic structure offers several advantages that facilitate the binding of noble gases and stabilize the formed NG adducts. The anionic superelectrophiles investigated by us are based on the closo-dodecaborate dianion scaffold. The record holder [B12(CN)11]- binds spontaneously almost all members of the NG family, including the very inert argon at room temperature and neon at 50 K in the gas phase of mass spectrometers. In this perspective, we summarize the argumentation for the advantages of anionic electrophiles in binding of noble gases and explain them in detail using several examples. Then we discuss the next steps necessary to obtain a comprehensive understanding of the binding properties of electrophilic anions with NGs. Finally, we discuss the perspective to prepare bulk ionic materials containing NG derivatives of the anionic superelectophiles. In particular, we explore the role of counterions using computational methods and discuss the methodology, which may be used for the actual preparation of such salts.

Comparison of computationally cheap methods for providing insight into the crystal packing of highly bromomethylated azobenzenes.

For five bromomethylated azobenzenes, namely (E)-[4-(bromomethyl)phenyl][4-(dibromomethyl)phenyl]diazene, C14H11Br3N2, (E)-1,2-bis[4-(dibromomethyl)phenyl]diazene, C14H10Br4N2, (E)-[3-(bromomethyl)phenyl][3-(dibromomethyl)phenyl]diazene, C14H11Br3N2, (E)-[3-(dibromomethyl)phenyl][3-(tribromomethyl)phenyl]diazene, C14H10Br4N2, and (E)-1,2-bis[3-(dibromomethyl)phenyl]diazene, C14H9Br5N2, the computationally cheap CLP PIXEL approach and CrystalExplorer were used for calculating lattice energies and performing Hirshfeld surface analysis via the enrichment ratios of atomic contacts. The procedures and caveats are discussed in detail. The findings from these tools are contrasted with the results of geometric analysis of the structures. We conclude that an energy-based discussion of the crystal packing provides substantially more insight than one based purely on geometry, as has so long been the custom in crystallography. In addition, we find a surprising shortage of halogen-halogen interactions in these highly bromomethylated compounds.

Evidence for an intrinsic binding force between dodecaborate dianions and receptors with hydrophobic binding pockets.

A gas phase binding study revealed strong intrinsic intermolecular interactions between dianionic halogenated closo-dodecaborates [B12X12](2-) and several neutral organic receptors. Oxidation of a tetrathiafulvalene host allowed switching between two host-guest binding modes in a supramolecular complex. Complexes of ß-cyclodextrin with [B12F12](2-) show remarkable stability in the gas phase and were successfully tested as carriers for the delivery of boron clusters into cancer cells.

Selective steroid recognition by a partially bridged resorcin[4]arene cavitand.

The partially bridged resorcin[4]arene cavitand featuring a cleft-shaped recognition site formed by two anti-quinoxaline bridges and four convergent HO-groups was prepared in three steps and characterised by X-ray crystallography; cavitand was found to be a selective receptor for steroidal substrates in CDCl3, with the best binding observed for steroids with a flat A-ring and two H-bonding sites on rings A and C/D.